Analog optical transmission has recently attracted renewed interest owing to the inherently low-noise characteristics of fiber optic systems. With the advent of single mode systems noise levels have been greatly reduced by eliminating modal noise within the fiber. Also, recent improvements in laser diodes have led to dramatically lower noise levels. With these advances in fiber optic technology, analog transmission becomes extremely attractive in applications where the desired signals at the terminal ends are analog (for example, commercial television). By eliminating the need for modulation conversion the terminal equipment is greatly reduced in complexity and cost.
One remaining obstacle to widespread use of analog optical transmission is the intrinsic nonlinearity of typical light emitting devices. Distortion of the signal caused by nonlinearities can be a problem, particularly in multichannel systems, where a high degree of linearity is required to prevent interchannel crosstalk.
Linearization of a laser diode or high bandwidth light emitting diode (LED) by direct feedback is generally not feasible due to the difficulty of controlling phase shift at the extremely high frequencies involved. However, other methods of linearization are possible. For example, a quasi-feed-forward technique by Straus and Szentesi, Electronics Letters, 13, 158 (1977), effectively linearized an LED by using a matched LED to cancel the distortion. Linearization of an LED by predistorting the input signal was reported by Asatani and Kimura, Electronics Letters, 13, 162 (1977) and IEEE Journal of Solid-State Circuits, SC-13, 133 (1978). Their method, using fixed predistortion, gave second and third harmonic distortion improvement of 28 dB and 31 dB respectively.